LeCroy WaveRunner Xi series Operator's Manual page 94

W R X S
AVE UNNER I
pctl Percentile
Definition:
Computes the horizontal data value that separates the data in a histogram such that the
population on the left is a specified percentage `xx' of the total population. When the threshold is
set to 50%, pctl is the same as hmedian.
Description:
The total population of the histogram is determ
each bin is summed until a bin that causes the sum to e
value is encountered. A ratio of the number of counts ne
popu lation is then determin
range is found, and returned as pctl.
Example:
The total population of a histogram is 100. The histogram range is divided into 20 bins and `xx' is
set to 25%. The population sum at the sixth bin from the left is 22. The population of the seventh
is 9 and its sub-range is 6.1 to 6.4 V. The ratio of counts needed for 25% population to total bin
population is:
•
3 counts needed / 9 counts = 1/3.
The value for pctl is:
•
6.1 volts + .33 * (6.4 6.1) volts = 6.2 volts.
pks Peaks
Definition:
The number of peaks in a histogram.
Description:
The instrument analyzes histogram data to identify peaks from background noise and histogram
binning artifacts such as small gaps.
Peak identification is a 3-step process:
1. The mean hei
calculated from this mean, where:
T1= mean + 2 sqrt (mean).
2. A second threshold is determined based on all populated bins under T1 in height, where:
T2 = mean + 2 * sigma, and where sigma is the standard deviation of all populated bins
under T1.
3. Once T2 is defined, the histogram distribution is scanned from left to right. Any bin that
crosses above T2 signifies the existence of a peak. Scanning continues to the right until one
bin or more crosses below T2. However, if the bins cross below T2 for less than a hundredth
of the histogram range, they are ignored, and scanning continues in search of peaks that
cross under T2 for more than a hundredth of the histogram range. Scanning goes on over
the remainder of the range to identify additional peaks. Additional peaks within a fiftieth of the
range of the populated part of a bin from a previous peak are ignored.
Note: If the number of bins is set too high, a histogram may have many small gaps. This
increases sigma and, thereby, T2. In extreme cases, it can prevent determination of a peak,
even if one appears to be present to the eye.
94
ERIES
ed fo r the bin. The horizontal value of the bin at that ratio point of its
ght of the histogram is calculated for all populated bins. A threshold (T1) is
ined. Scanning from left to right, the population of
qual or exceed `xx'% of the population
eded for `xx'% population/total bin
WRXi-OM-E Rev C